Signals - may be external (alerting
about environmental conditions) or internal (alerting body to internal
conditions); signals are a form of energy - may be chemical, light,
heat/cold, touch, sound, etc.

Receptor - must be designed for
signal. The receptor for light is a pigment; the receptor for a
chemical is typically a protein (often embedded in membrane)

Transducing mechanism - refers to
the complex response system by which activating the receptor initiates the
response.

II.
Internal Chemical Signals (= hormones) Hormones are defined as: organic compound,
active in low concentration, that is produced in one part of the cell/organism
and has it action in another. Thus, there is a separation of the site of
production and synthesis. The degree of separation varies: in
plants, usually little separation; in animals - in some cases there is little (autocrine),
minimal (paracrine) or lots (endocrine).

A. Plants - check out
the Case Studies in
phototropism and fruit set in soybeans
B. Animals - check out the secretin example in dogs (Case
Study)

IV.
Criteria to establish that a chemical is a hormone (check out the
Case
Study)
Essentially, four criteria need to be met:

exogenous application causes the
response

lowering endogenous levels prevents
the response

lowering endogenous and then adding
the chemical exogenously returns the response

endogenous levels should increase
before the response is detected

V.
Chemistry of Hormones A variety of molecules are used as hormones.
These include:

polypeptides (=proteins)

steroids

terpenoids - built from isoprene

others - amino acids, etc.

VI.
Mechanism of Action

A. General
Hormones typically bind to receptors to initiate a response.
The receptors are usually proteins and can be located in the cell membrane (for
hydrophilic, water-soluble hormones) or somewhere in the interior of the cell
(lipid-soluble hormones like steroids). The receptor then initiates a
series of responses that amplifies the hormonal signal ultimately resulting in
an increase in transcription and/or translation and/or protein activation.
The amplification process often involves protein kinases that add/remove a
phosphate from an enzyme to activate/inactivate the enzyme (enzyme-inactive + Pi
→ enzyme-active). A general scheme includes:

B. Example 1 - Estradiol
Estradiol is a steroidal sex hormone and readily crosses
membranes. Estradiol enters the nucleus and binds to a protein receptor
(we know it is a protein because when a cell extract is treated with proteases
there is no response) the activated protein/hormone complex binds to DNA at the
promoter region activating transcription and ultimately translation.
Proteins that bind to DNA typically have regions called zinc-fingers.

C. Example 2
The water-soluble hormones, like noradrenaline, typically
bind to a protein receptor embedded in the membrane (transmembrane protein).
The membrane protein then activates a cascade/amplification of events that can
either: